Generate random `PaymentId` for outbound offers

Generate random `PaymentId` for outbound offers

Previously, we'd deterministically derive the `offer_hash` as a
`PaymentId` for outbound BOLT 12 payments. However, as offers may be
paid multiple times, this could result in collisions in our
`outbound_payments` store.

Here, we therefore use random `PaymentId`s to avoid collisions, even if
offers are paid multiple times.